Conventionally, a phase difference element (for example, a wave plate) has been made from an inorganic optical single crystal, such as a quartz crystal, and a polymeric stretched film. However, although the inorganic optical single crystal has excellent performance, durability and reliability as a phase difference element, it has the disadvantages of high cost of raw materials and their processing. The polymeric stretched film, on the other hand, has the disadvantages of low durability since it tends to deteriorate due to heat and UV radiation.
An oblique deposited film having an oblique columnar structure, by the way, has been expected to be used as an oblique deposited phase difference element. It is based on that any phase differences can be set by adjusting the film thickness in principle, enlargement of the film area is relatively easy, and low cost and mass production can be achieved.
The below Patent Literatures 1 to 7 describe oblique deposited phase difference elements. In Patent Literatures 1 to 3 among them, Ta2O5 with excellent heat resistance is used as a deposition material. This Ta2O5 is a material having a high refractive index (approximately 2.2) and the oblique deposited film made therefrom has high birefringence, whereas not only Ta2O5 but also the optical thin film formed by oblique deposition has lower transmittance of light due to larger optical loss than the optical thin film formed by ordinary deposition techniques. Moreover, the oblique deposited phase difference element is required to have a film thickness of one to several μm to serve as a wave plate, and thicker film thickness is more sensitive to the optical loss.
Patent Literature 8 discloses a method for laminating a number of oblique deposited thin films and front deposited thin films to decrease the optical loss. In this method, each oblique deposited film is thinned to stabilize the acicular structure of a deposition substance, and the front deposited film with high density is disposed between oblique deposited films to increase mechanical strength of the film.
In addition, Patent Literature 9 describes a method in combination with a plasma source or an ion source in a deposition apparatus 1, the method including applying a deposition substance in a high energy state to a substrate by taking advantages of plasma formed in the apparatus to increase the adhesion of the deposition substance to the substrate and thereby decrease the optical loss.
The methods disclosed in Patent Literatures 8 and 9, however, have the problems of a longer lead time, limited deposition apparatus and so on because the optical loss of the film is decreased by a complex film configuration and apparatus structure.
Also, with regard to an oblique deposited phase difference element as a phase difference element having excellent heat resistance, there has been the problem of spectral transmittance properties in applying to a wave plate due to a demand for increased luminance in a projector for years. In particular, for the wave plate required to have a thickness of oblique deposited film ranging from one to several μm, transmittance is markedly decreased in the wavelength range of from 400 to 500 nm, thereby causing the problem of, for example, decreased luminance of blue light when applied to a liquid crystal projector.